Evidence for the Unexpected Associative Displacement of Adenosyl by Cyanide in Coenzyme B12

• Published in: Inorganic chemistry 1997-07, Vol.36 (15), p.3216-3222
• Main Authors: Brasch, Nicola E, Hamza, Mohamed S. A, van Eldik, Rudi
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic961312z

The reaction of coenzyme B12 (adenosylcobalamin) with cyanide has been reinvestigated in detail using spectroscopic and kinetic techniques. It has been shown that this reaction proceeds in one kinetically observable step, contradicting previous findings, with rate-determining attack of the first cyanide (k = (7.4 ± 0.1) × 10-3 M-1 s-1, 25.0 °C, I = 1.0 M (NaClO4)). The activation parameters were found to be ΔH ⧧ = 53.0 ± 0.6 kJ mol-1, ΔS ⧧ = −127 ± 3 J mol-1 K-1 and ΔV ⧧ = −10.0 ± 0.4 cm3 mol-1, suggesting an associative displacement mechanism. It is postulated that attack of the first cyanide occurs at the β-(5‘-deoxy-5‘-adenosyl) site rather than at the α-dimethylbenzimidazole site.